Earth Planets Space, Vol. 61 (No. 1), pp. 119-124, 2009
Aleksey V. Smirnov
Department of Geological and Mining Engineering and Sciences, Michigan Technological University, Houghton, MI 49931, USA
(Received December 28, 2007; Revised February 29, 2008; Accepted March 11, 2008; Online published January 23, 2009)
Magnetic measurements at cryogenic temperatures (<300 K) proved to be useful in paleomagnetic and rock magnetic research, stimulating continuous interest to low-temperature properties of magnetite and other magnetic minerals. Here I report new experimental results on a grain size dependence of the ratio (RLT) between a low-temperature (20 K) saturation isothermal remanent magnetization (SIRM) imparted in magnetite after cooling in a 2.5 T field (field cooling, FC) and in a zero field environment (zero field cooling, ZFC). Synthetic magnetite samples ranged in mean grain size from 0.15 to 100 μm, representing nearly single-domain (SD), pseudosingle-domain (PSD), and multidomain (MD) magnetic states. The RLT ratio monotonically increases from 0.58 to 1.12 with the decreasing mean grain size, being close to unity for PSD grains (0.15-5 μm) and smaller than unity for MD magnetite (12-100 μm). The RLT ratio of 1.27 is observed for acicular magnetite characterized by nearly SD behavior. These observations indicate that within the range of ∼0.15 to ∼5 μm, the low-temperature SIRM may be higher than that expected from "normal" magnetic domain wall displacement. Such a behavior can be caused by the presence of a SD-like component in the magnetization of these grains, which origin, however, is uncertain. The natural rocks containing nearly stoichiometric magnetite manifest a dependence of the RLT ratio on magnetic domain state identical to that observed from synthetic magnetites. Therefore, the comparison of FC SIRM and ZFC SIRM at very low temperatures may allow a crude estimate of magnetic domain state in some magnetite-bearing rocks, such as shallow mafic intrusions or some marine sediments.
Key words: Magnetite, Verwey transition, twinning, remanent magnetization, zero field cooling.